Experimental and theoretical analysis of spalling in deep hard rock tunnels with different arch structures

To study the spalling characteristics and induction mechanism of tunnels with different arch structures, a series of true triaxial tests were conducted on three types of cubic granite specimens with a through-going arched hole and analytical stress solutions were obtained for the three arch sections. Test results show that the failure process of specimens includes four periods: a quiet period, a buckling deformation period, a gradual buckling and exfoliation period of rock fragments, and a V-shaped notch formation period. Specimens with different arch structures are listed in a descending order as the three-centered arch with f/B (arch rise/tunnel width) = 1/3 (II), three-centered arch with f/B = 1/4 (III), and semi-circular arch with f/B = 1/2 (I) according to the size of rock fragments. Before the spalling of arched tunnels, the acoustic emission (AE) count is low, while the AE count and cumulative AE count are high during spalling. Stress analysis results show that as the vertical stress rises, the arch feet of tunnels undergo initial failure at first due to stress concentration; spalling cracks appear in both side walls due to compressive stress concentration, which induce spalling failure; tensile cracks initiate in the vault and invert due to tensile stress concentration. The sequence of spalling in tunnels with different arch structures varies due to a change in section structure. Therefore, the quality of section structures can be evaluated according to the stress distribution characteristics of surrounding rocks. The vertical stress inducing the initial failure of tunnel surrounding rocks increases gradually in different arch structures from the II, III, to I; the failure severity of surrounding rocks decreases in different arch structures, also from II, III, to I. In addition, the stress analysis results also verify the accuracy of the test results. In practical engineering applications, to consider both the section utilization ratio and control of spalling in surrounding rocks, selecting tunnel sections with f/B = 1/4 is more economic and safer than that of f/B = 1/3.